1. Field of the Invention
The present invention relates to a MOS transistor with a deformable gate. Such a transistor may especially be used as a pressure sensor, an acceleration sensor, or as a resonator.
2. Discussion of the Related Art
FIG. 1 is a perspective view of a known MOS transistor with a deformable gate. This type of transistor is described in patent WO03/078299 of the Ecole Polytechnique Fédérale de Lausanne entitled “PROCESS FOR MANUFACTURING MEMs”, published on Sep. 25, 2003. This transistor is formed in and above a semiconductor substrate 1. A gate beam 2 bears at each end on pillars 3 and 4 placed on substrate 1. Source/drain areas 5 and 6 formed in substrate 1 are placed on each side of beam 2. The substrate portion located under beam 2 comprises a channel area 7.
FIGS. 2 and 3 are cross-section views of FIG. 1 along the longitudinal axis of gate beam 2. In the quiescent state, as shown in FIG. 2, beam 2 is horizontal. The beam is capable of deforming under the action of a mechanical, electrostatic, or electromagnetic force. When the beam deforms, it curves in an “arc”, as shown in FIG. 3. The central portion of gate beam 2 is close to channel area 7 while the ends of the beam are remote from the channel area.
The beam comprises a conductive layer that can be set to a determined voltage and forms the gate of the MOS transistor having, as a source and drain, areas 5 and 6 and, as a channel area, area 7.
When the beam is horizontal, the gate-substrate capacitance is small. The transistor then exhibits a high threshold voltage. The more the beam deforms and comes closer to the substrate, the more the transistor threshold voltage decreases. Accordingly, for a given biasing of the gate and of the source/drain areas, the more gate beam 2 is deformed, the higher the current flowing through the transistor. Thus, the motions of the gate beam translate as current variations through the transistor.
However, the current flowing through channel area 7 is not uniform on this entire area. Since the distance between beam 2 in the deflected state and channel area 7 is smaller in the central portion of the channel area than outside of this area, close to pillars 3 and 4, the current density is maximum at the center of the channel area and minimum close to the pillars. Accordingly, the major part of the current flowing through the transistor runs through the central portion of the channel. The lateral portions then play a minor function in the detection of the beam deformation.